Operation of lean burn engines, e.g. diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy and have relatively low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their durability and their ability to generate high torque at low speed.
From the standpoint of emissions, however, diesel engines present problems more severe than their spark-ignition counterparts. Emission problems relate to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others. NO and NO2 are of concern because they are believed to participate in photo-chemical smog formation through a series of reactions in the presence of sunlight and hydrocarbons. Furthermore, NO2 is a significant contributor to acid rain, it has a high potential as an oxidant, and is a strong lung irritant. Particulates (PM) are also connected with respiratory problems. However, as engine operation modifications are made to reduce particulates and unburned hydrocarbons from diesel engines, the NO and NO2 emissions tend to increase.
Effective abatement of NOx from lean burn engines is difficult to achieve because high NOx conversion rates typically require fuel-rich (i.e. high-reductant) conditions. Conversion of the NOx component of exhaust streams to innocuous components generally requires specialized NOx abatement strategies for operation under fuel lean conditions
Oxidation catalysts comprising a precious metal dispersed on a refractory metal oxide support are used in treating the exhaust of diesel engines in order to convert both hydrocarbon and carbon monoxide gaseous pollutants to carbon dioxide and water. Typically, diesel oxidation catalysts (DOC) are formed on ceramic or metallic substrate carriers (such as flow-through monolith carriers) upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC, CO, and the soluble organic fraction (SOF) of particulate matter to carbon dioxide and water, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) promote the oxidation of NO to NO2.
High surface area refractory metal oxides are often employed as a support for many of the catalytic components. For example, high surface area alumina materials, also referred to as “gamma alumina” or “activated alumina,” used with oxidation catalysts typically exhibit a BET surface area in excess of 60 m2/g, and often up to about 200 m2/g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa, and theta alumina phases. Refractory metal oxides other than activated alumina may be utilized as a support for at least some of the catalytic components in a given catalyst. For example, bulk ceria, zirconia, alpha-alumina, and other materials are known for such use. Although many of these materials have a lower BET surface area than activated alumina, that disadvantage tends to be offset by the greater durability of the resulting catalyst or a beneficial interaction with precious metal deposited on the support.
Catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation because the engine exhaust is not at a temperature sufficiently high enough for efficient catalytic conversion of noxious components in the exhaust.
Oxidation catalysts comprising a platinum group metal (PGM) dispersed on a refractory metal oxide support are known for use in treating exhaust gas emissions from diesel engines. Platinum (Pt) is an effective metal for oxidizing CO and HC in a DOC after high temperature aging under lean conditions and in the presence of fuel sulfur. On the other hand, palladium (Pd) or Pd-rich diesel oxidation catalysts typically show higher light-off temperatures for oxidation of CO and HC, especially when used to treat exhaust containing high levels of sulfur (from high sulfur containing fuels). “Light-off” temperature for a specific component is the temperature at which 50% of that component reacts. DOCs comprising a large percentage of Pd may poison the activity of Pt to convert CO and HCs and may also make the catalyst more susceptible to sulfur poisoning. These characteristics have typically prevented the use of Pd-rich oxidation catalysts in lean burn operations, especially for light duty diesel application where engine temperatures remain below 250° C. for most driving conditions.
As mentioned above, the primary function of diesel oxidation catalysts in diesel vehicle applications has been to oxidize carbon monoxide and hydrocarbons to carbon dioxide and water. Recent implementation of selective catalytic reduction catalysts (SCR) in the exhaust systems of diesel vehicles in order to meet NOx emission legislation, however, has required the DOC to also function as an efficient NO oxidation catalyst. While SCR catalyst systems have been shown to maximize NOx reduction performance when the ratio of NO2 to NO in the exhaust is approximately 50%, typical concentrations of NO2 in the exhaust are much lower. Due to the high temperatures of combustion, the primary NOx component exiting the engine is NO.
Furthermore, DOC catalysts based on Pt/Pd are notoriously poor for oxidizing NO to NO2. This is especially true for DOCs containing significant quantities of Pd (e.g. 2:1 or 1:1 weight ratio of Pt to Pd). For DOC applications, Pt and Pd are the preferred precious metals for oxidation of CO and HC present in diesel engine exhaust, and the choice of these active metals is due to a combination of performance (i.e. mixtures of Pt and Pd have improved performance compared to Pt and Pd alone) and cost (i.e. the price of Pd is significantly cheaper than that of Pt). However, as more Pd is added to the DOC, the NO oxidation performance declines, and SCR catalysts located downstream of the DOC are exposed to lower than optimal levels of NO2. While the NO oxidation performance of the DOC can be increased by increasing the quantity of Pt (and correspondingly decreasing the amount of Pd), this is not a cost effective solution due to the high price of platinum relative to palladium. In addition, if the Pt/Pd ratio becomes too large, the CO and HC oxidation activity may actually decline.
As emissions regulations become more stringent, there is a continuing need to develop diesel oxidation catalysts systems that provide improved performance, for example, improved NO oxidation capability. There is also a need to utilize components of DOCs, for example, Pt and Pd, as effectively as possible.
Accordingly, it would be desirable to provide improved catalyst materials including carriers for platinum and palladium that exhibit improved performance.